Standard safety valves for industrial applications are commonly constructed in a manner disclosed generally, for example, in U.S. Pat. No. 2,628,632. In such arrangements, the valve typically includes a flow passage extending from an inlet to an outlet on an axis offset with respect to the inlet. At an intermediate location of the passage, a closure disc cooperates with an annular seat to open and close the valve to fluid flow. Urging the disc toward a closed position by imposing a predetermined loading force against the topside of the disc is usually achieved by means of a compressed coil spring acting in opposition to the fluid force of the system being monitored that tends to urge the disc on its underside toward the open position. The magnitude of force imposed by the compressed spring is generally preadjusted to a customer's specification for the set point at which a value of fluid pressure to be relieved can initiate movement of the disc toward the open valve position.
Valves of the foregoing type are typically utilized for relief of fluid pressures on the order of 50 to 2000 psig and at temperatures usually less than about 1000.degree. F. Under such conditions, set point operation of the valve is reliably predictable pursuant to the government codes regulating performance of these valves. However, under operating conditions of relatively low pressure, on the order of less than about 50 psig, and high fluid temperature, on the order of 1000.degree. F. and above, thermal conductivity from the fluid median within the valve body tends to introduce physical property changes in the coil spring which in turn alters the spring load previously set. These load changes that are attributed to spring force variations caused by thermal expansion and modulus of elasticity changes of the spring materials as a function of temperature, have been found to cause operational instability of the valve by adversely affecting the intended set point previously established.
Prior but unsuccessful attempts to resolve the foregoing have largely centered around modified spring designs or inverted mountings to remove intrinsic component weight effects. However, despite recognition of the problem, a more satisfactory solution therefor has not heretofore been known.